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Search Results: 1 - 10 of 21305 matches for " PENG Qun "
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Chemical scissors cut phosphorene nanostructures and their novel electronic properties
Xihong Peng,Qun Wei
Physics , 2014, DOI: 10.1088/2053-1591/1/4/045041
Abstract: Phosphorene, a recently fabricated two dimensional puckered honeycomb structure of phosphorus, showed promising properties for applications of nano-electronics. In this work, we report our findings of chemical scissors effects on phosphorene, using first principles density functional theory methods. It was found that several chemical species, such as H, F, Cl and OH group, can act effectively as scissors to cut phosphorene. Phosphorus chains and nanoribbons can be obtained using different surface coverage of the chemical species. The scissor effects of these species are resulted from their strong chemical bonds with the P atoms. Species such as O, S and Se were not able to cut phosphorene nanostructures due to their lack of strong binding with P. The electronic structure calculations of the produced P-chains reveal that the saturated chain is an insulator while the pristine chain demonstrates a Dirac point at X with a Fermi velocity of 8*10E5 m/s. The obtained zigzag phosphorene nanoribbons show either metallic or semiconducting behaviors, depending on the treatment of the edge P atoms.
Superior mechanical flexibility of phosphorene and few-layer black phosphorus
Qun Wei,Xihong Peng
Physics , 2014, DOI: 10.1063/1.4885215
Abstract: Recently fabricated two dimensional (2D) phosphorene crystal structures have demonstrated great potential in applications of electronics. Mechanical strain was demonstrated to be able to significantly modify the electronic properties of phosphorene and few-layer black phosphorus. In this work, we employed first principles density functional theory calculations to explore the mechanical properties of phosphorene, including ideal tensile strength and critical strain. It was found that a monolayer phosphorene can sustain tensile strain up to 27% and 30% in the zigzag and armchair directions, respectively. This enormous strain limit of phosphorene results from its unique puckered crystal structure. We found that the tensile strain applied in the armchair direction stretches the pucker of phosphorene, rather than significantly extending the P-P bond lengths. The compromised dihedral angles dramatically reduce the required strain energy. Compared to other 2D materials such as graphene, phosphorene demonstrates superior flexibility with an order of magnitude smaller Young modulus. This is especially useful in practical large-magnitude-strain engineering. Furthermore, the anisotropic nature of phosphorene was also explored. We derived a general model to calculate the Young modulus along different directions for a 2D system.
A Non-Krylov subspace Method for Solving Large and Sparse Linear System of Equations
Wujian Peng,Qun Lin
Mathematics , 2015,
Abstract: Most current prevalent iterative methods can be classified into the so-called extended Krylov subspace methods, a class of iterative methods which do not fall into this category are also proposed in this paper. Comparing with traditional Krylov subspace methods which always depend on the matrix-vector multiplication with a fixed matrix, the newly introduced methods(the so-called (progressively) accumulated projection methods, or AP (PAP) for short) use a projection matrix which varies in every iteration to form a subspace from which an approximate solution is sought. More importantly an accelerative approach(called APAP) is introduced to improve the convergence of PAP method. Numerical experiments demonstrate some surprisingly improved convergence behavior. Comparison between benchmark extended Krylov subspace methods(Block Jacobi and GMRES) are made and one can also see remarkable advantage of APAP in some examples. APAP is also used to solve systems with extremely ill-conditioned coefficient matrix (the Hilbert matrix) and numerical experiments shows that it can bring very satisfactory results even when the size of system is up to a few thousands.
Recent progress on the structure of Ser/Thr protein phosphatases
BaiJing Wang,Peng Zhang,Qun Wei
Science China Life Sciences , 2008, DOI: 10.1007/s11427-008-0068-y
Abstract: PP1, PP2A and PP2B, belonging to the PPP family of Ser/Thr protein phosphatases, participate in regulating many important physiological processes, such as cell cycle control, regulation of cell growth and division regulation, etc. The sequence homology between them is relatively high, and tertiary structure is conserved. Because of the complexity of the structure of PP2A and the diversity of its regulatory subunits, its structure is less well known than those of PP1 and PP2B. The PP2A holoenzyme consists of a heterodimeric core enzyme, comprising a scaffolding subunit and a catalytic subunit, as well as a variable regulatory subunit. In this study, the subunit compositions, similarities and differences between the Ser/Thr protein phsphatases structures are summarized.
Recent progress on the structure of Ser/Thr protein phosphatases

BaiJing Wang,Peng Zhang,Qun Wei,

中国科学C辑(英文版) , 2008,
Abstract: PP1, PP2A and PP2B, belonging to the PPP family of Ser/Thr protein phosphatases, participate in regulating many important physiological processes, such as cell cycle control, regulation of cell growth and division regulation, etc. The sequence homology between them is relatively high, and ter- tiary structure is conserved. Because of the complexity of the structure of PP2A and the diversity of its regulatory subunits, its structure is less well known than those of PP1 and PP2B. The PP2A holoen- zyme consists of a heterodimeric core enzyme, comprising a scaffolding subunit and a catalytic sub- unit, as well as a variable regulatory subunit. In this study, the subunit compositions, similarities and differences between the Ser/Thr protein phsphatases structures are summarized.
CRLBs for WSNs localization in NLOS environment
Huang Jiyan,Wang Peng,Wan Qun
EURASIP Journal on Wireless Communications and Networking , 2011,
Abstract: Determination of Cramer-Rao lower bound (CRLB) as an optimality criterion for the problem of localization in wireless sensor networks (WSNs) is a very important issue. Currently, CRLBs have been derived for line-of-sight (LOS) situation in WSNs. However, one of major problems for accurate localization in WSNs is non-line-of-sight (NLOS) propagation. This article proposes two CRLBs for WSNs localization in NLOS environment. The proposed CRLBs consider both the cases that positions of reference devices (RDs) are perfectly or imperfectly known. Since non-parametric kernel method is used to build probability density function of NLOS errors, the proposed CRLBs are suitable for various distributions of NLOS errors. Moreover, the proposed CRLBs provide a unified presentation for both LOS and NLOS environments. Theoretical analysis also proves that the proposed CRLB for NLOS situation becomes the CRLB for LOS situation when NLOS errors go to 0, which gives a robust check for the proposed CRLB.
Division of the two-qubit Hilbert space according to the entanglement sudden death under composite noise environment
Peng Li,Qun Zhang,J. Q. You
Physics , 2008, DOI: 10.1103/PhysRevA.79.014303
Abstract: We show theoretically that according to the disentanglement behavior under composite noise environment, the Hilbert space of a two-qubit system can be divided into two separate parts: a 3-dimensional subspace in which all states disentangle asymptotically, and the rest in which all states disentangle abruptly. The violation of additivity for entanglement decay rates under weak noises [see, PRL 97, 140403 (2006)] therefore can be explained in terms of such division of the Hilbert space.
Strain engineered direct-indirect band gap transition and its mechanism in 2D phosphorene
Xihong Peng,Andrew Copple,Qun Wei
Physics , 2014, DOI: 10.1103/PhysRevB.90.085402
Abstract: Recently fabricated two dimensional (2D) phosphorene crystal structures have demonstrated great potential in applications of electronics. In this work, strain effect on the electronic band structure of phosphorene was studied using first principles methods. It was found that phosphorene can withstand a surface tension and tensile strain up to 10 N/m and 30%, respectively. The band gap of phosphorene experiences a direct-indirect-direct transition when axial strain is applied. A moderate -2% compression in the zigzag direction can trigger this gap transition. With sufficient expansion (+11.3%) or compression (-10.2% strains), the gap can be tuned from indirect to direct again. Five strain zones with distinct electronic band structure were identified and the critical strains for the zone boundaries were determined. The origin of the gap transition was revealed and a general mechanism was developed to explain energy shifts with strain according to the bond nature of near-band-edge electronic orbitals. Effective masses of carriers in the armchair direction are an order of magnitude smaller than that of the zigzag axis indicating the armchair direction is favored for carrier transport. In addition, the effective masses can be dramatically tuned by strain, in which its sharp jump/drop occurs at the zone boundaries of the direct-indirect gap transition.
Edge effects on the electronic properties of phosphorene nanoribbons
Xihong Peng,Qun Wei,Andrew Copple
Physics , 2014, DOI: 10.1063/1.4897461
Abstract: Two dimensional few-layer black phosphorus crystal structures have recently fabricated and demonstrated great potential in applications of electronics. In this work, we employed first principles density functional theory calculations to study the edge effects and quantum confinement on the electronic properties of the phosphorene nanoribbons (PNR). Different edge functionalization groups, such as H, F, Cl, OH, O, S, and Se in addition to a pristine case, were studied for a series width of the ribbon up to 3.5 nm. It was found that the armchair-PNRs (APNRs) are semiconductors for all edge groups considered in this work. However, the zigzag-PNRs (ZPNRs) show either semiconductor or metallic behavior in dependence on their edge chemical groups. Family I edges (H, F, Cl, OH) form saturated bonds with P atoms and the edge states keep far away from the band gap. However, Family II edges (pristine, O, S, Se) form weak unsaturated bonds with the pz orbital of P atoms and bring edge states within the band gap. These edge states of Family II ribbons present around the Fermi level within the band gap, which close up the band gap of the ZPNRs. For the APNRs, these edge states are at the bottom of the conduction band and result in a reduced band gap.
Improved Degree Search Algorithms in Unstructured P2P Networks
Guole Liu,Haipeng Peng,Lixiang Li,Yixian Yang,Qun Luo
Mathematical Problems in Engineering , 2012, DOI: 10.1155/2012/923023
Abstract: Searching and retrieving the demanded correct information is one important problem in networks; especially, designing an efficient search algorithm is a key challenge in unstructured peer-to-peer (P2P) networks. Breadth-first search (BFS) and depth-first search (DFS) are the current two typical search methods. BFS-based algorithms show the perfect performance in the aspect of search success rate of network resources, while bringing the huge search messages. On the contrary, DFS-based algorithms reduce the search message quantity and also cause the dropping of search success ratio. To address the problem that only one of performances is excellent, we propose two memory function degree search algorithms: memory function maximum degree algorithm (MD) and memory function preference degree algorithm (PD). We study their performance including the search success rate and the search message quantity in different networks, which are scale-free networks, random graph networks, and small-world networks. Simulations show that the two performances are both excellent at the same time, and the performances are improved at least 10 times.
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